Capacity Analysis of an MISO Free-Space Optical Communications Assisted by Concave ORIS

This paper explores the ergodic channel capacity of multiple-input single-output (MISO) free-space optical (FSO) communication systems, assisted by (optical) re-configurable intelligent surfaces [(O)RIS], made of concave reflectors. On the one hand, RIS technology mitigates dead zones in communication systems. Additionally, it increases the data rate and communication range, enhances the communication channel by making it intelligent, and improves the system's capacity. Finally, the RIS technology improves the spectrum and energy efficiencies of the considered systems. On the other hand, transmitting diversity mitigates deep fade and helps to achieve beamforming to regulate the beam sent in a specific direction. Finally, multiple light sources help to send different versions of the same information at other time slots. Furthermore, compared to flat reflectors, concave mirrors provide economic advantages enabled by their natural shape, which helps converge the impinging light beams into the same focal point. In this paper, we harness the full potential of ORIS and MISO technologies in an FSO system by exploiting the hollow of concave reflectors to focus the reflected beams on a single user. We derive an approximated closed-form expression, provide results of the proposed ORIS-aided FSO systems' ergodic channel capacity, and discuss the suitable type of concave reflector. These results show that all types of concave mirrors provide similar results except when the thickness of the reflector is large enough to impact the reflected light.